Pregnancy Timeline & Paternity Identification: Understanding Conception Dates

Question
Hello, I am trying to determine when I conceived and ensure which of the two men involved are the father. My lmp was September 9th, however I have not been tracking the length of my cycles and think that I may have irregular periods that differ monthly by a few days to a week but not 100% sure of this. I had intercourse with man #1 on either September 18 or 19. Did not sleep with either for days - until September 25 which is when I had sex with man #2. Man #2 and I had sex regularly after that up until I discovered my pregnancy. On October 18th I was told I was 5-6 weeks pregnant based on HCG levels. I had my first ultrasound on December 19(abdominal ) and it read that I was 13 weeks 5 days pregnant.  Any professional and expert advice would be appreciated!

Answer
Unfortunately, you'll need a paternity test to know for certain.  Based on your LMP, you most likely conceived around Sept. 23 but according to your second ultrasound, you most likely conceived around Sept. 28.  Since sperm can live in the reproductive tract for up to 5 days, either man could be the father. Without more accurate measurements or a history of your cycles, there's no way to say for certain without a paternity test.